text:
Why does the F16 HUD give you one number for your speed and the analogue gauge another?
Why does the F16 HUD give you one number for your speed and the analogue gauge another different number?

Could you add an example image to show the two different numbers?

The primary airspeed indication is in the HUD. You can select CAS (calibrated airspeed), TAS (true airspeed) and GND SPD (ground speed) on the HUD panel on the right console.
The backup airspeed indicator (the analogue gauge you are referring to) is located below the ICP (integrated control panel) on the center console. It can indicate from 80 kt to 800kt with a scale in 10 kt increments up to 400 kt and then 50 kt increments up to 800 kt. It is a rather basic analogue instrument providing IAS (indicated airspeed) only.
As the scale is very coarse (higher precision is not needed) you won't be able to see a noticeable difference when the HUD is switched to CAS. Most probably the HUD was switched to TAS or GND SPD. But without further information or an example photo from your side it is hard to tell.


text:
Where can I find the planned and flown paths for certain flight routes?
Are there official definitions of air traffic corridor pathing? Specifically European & Mediterranean routes.
I'm working on an application that needs this input data to perform an analysis. I and others in my company have been unable to find anything, and I really don't want to have to derive something approximating it myself if I don't have to.
Edit: Since what I'm asking for doesn't seem to be very clear I'll try a different way.
When an commercial jet starts a flight from one airport to another. I assume it had a generally predetermined path (or options of paths). These are not straight shortest path between two points on an ellipsoid so the data at OpenFlights.org isn't helpful, as it is not accurate enough. Obviously though no one can fully predict exactly where an aircraft will be in the future because that changes in respect to many different unknown variables, but I need something more like this, which was done tracking one commercial jet from one airport to another which includes no amount of variation, and it takes a long time to do this live.
Ideally I could get the general routes that these individual flights tend to follow, but to get an approximation of the actual routes between airports would be a whole lot better than I am know.
In a perfect world there would be lat/lon/alt/ pairings of the paths with some sort of standard deviation on those points. I doubt that exists but anything that could get me closer to that would be excellent.
Paying for something is not necessarily a problem, as long as I can prove it to my boss that it is what we need.

Are there official definitions of air traffic corridor pathing please try to rephrase this.  Are you asking if there are defined paths (routes)?

What do you need it for? Perhaps if you explained, it would shed some light on what you actually need.

There are airways (that is the term you should probably be searching for) defined, but they are not always used.

Here are charts of airways for Eurocontrol. With the availability of GPS navigation aircraft can now fly more direct routes so fewer aircraft are using airways for navigation.

@ simon. I may not be using the best terms. I'm asking, "What are the defined routes?" and/or "Where can I find them?".

@Jan Hudec. I can't share what exactly I need them for, but I need to be able to perform analysis on an estimation of where aircraft should be traveling at an undetermined future time if that helps any.

@ TomMcW. I think that might be in the right direction, but without actual lat/lon definitions, I'm not sure it would be any less trouble than me making my own interpolations from live flight data.

You might try Navigraph as a start.  There are a course other providers but this is one I know.  It contains every airway in the world. Here's an example for an airway called B330 to see wha AIRAC data looks like. The difficult part is to associate flights with airways, especially as the same aircaft, operated by the same airline, can use different airways from day to day and generally, not predictable in advance.

Related question? Does official source of data about air routes exists?

@fooot yes that is a related question, but it doesn't have an answer.

@simon. Thank you. I'll look into that more, it looks promising. My company's web filter won't let me go to the second link due to "adult content"... And that is why I'm trying to find all the possible routes, so that the information can be preprocessed and a lookup done when the important time comes.

I've just realised the image misses something.  The AIRAC data contains the lat long of every waypoint along the route.

Here's an extract from an actual AIRAC data file. I've separated with pipes. Each group is airway, waypoint index, waypoint id, lat, long.  N163 001 MTG 43.386306 5.086833 |
N163 002 JULEE 43.012778 5.633333 |
N163 003 EPOLO 42.882778 6.534722 |
N163 004 OMARD 43.104722 7.591111

That looks like what I need. I will look into it more in depth and get back. Early next week hopefully (unless work pulls me onto something else...). thank you very much. This has probably been the best lead I've gotten since starting to look a few weeks ago.

A good starting point would be the Wikipedia article on flight planning. You may also look at sites that propose to share known routes between airports, like Flight Plan Database.

@mins Thank you. I'll look into that as well.

@Simon Still waiting to hear back from Navigraph...

Just an update: Navigraph told me their data is only for non-professional fligh simulation use and directed me to their supplier Jeppesen, Inc. They also noted a site called skyvector.com which definitely has the data, but I haven't had time to see if I could extract it from there yet.

Yes, depending.....
On FlightAware.com you can see the routing of a flight after it's flown, and the same flight the next day will probably file for the same route.
On FltPlan.com you can find typical routes that a plane is likely to be cleared on and have actually been cleared on over time. Pretty accurate, but you probably need flying skills to interpret it, or get training on understanding routing, airport ICAO codes, nav fixes, departures arrival and enroute routing.

If my memory serves flightaware has an api.  You might be able to get the flights paths from it.

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text:
What was the first monoplane airliner with a full cantilever wing (no struts)?

By Towpilot - Own work, CC BY-SA 3.0, Image source
By the mid 1930s, the gold standard of airliner construction was achieved: the DC-3 was a full aluminium aircraft with monocoque construction and a cantilever wing. But which aircraft was the first airliner with a full cantilever monoplane wing, no struts?

I would argue that the Boeing P-247 was the first to arrive at the gold standard. The DC-2 was a direct competition and took a lot of "inspiration" from the Boeing design, and the DC-3 was just a bigger version of the DC-2.

Yes indeed, the P247 was earlier, but the sheer numbers of the DC-3 are proof of the pudding I reckon.

When your owner is United Airlines and they don't like the competition to fly the same aircraft, your production run can become rather small. The DC-3 came early enough to be in time for the war and, being more advanced and bigger, saw higher production numbers.

I have heard of a Boeing model 247 but have not ever heard of a P-247.  But a google search  for P-247 turns up this -- https://modelplanes.de/luftwaffe/jaeger-luftwaffe/dornier-do-p-2476-unicraft-resin/

What about the Junkers J-1?  Nothing earlier comes to mind.
https://en.wikipedia.org/wiki/Junkers_J_1  -- An airliner for one person I guess

@quietflyer Yes indeed, Boeing 247.

Not the first, but probably the only cantilever-wing other than the Junkers F13, was the Fokker F.III, first flying Amsterdam-London on 14 April 1921, piloted by English pilot Hinchcliffe. Its fuselage was constructed of steel tubes and covered with fabric, and the wing was made entirely of wood, the outer covering being plywood.
The Junkers was of more modern construction, using corrugated duraluminium for the skin and an aluminium support structure. Both Anthony Fokker and his factory GM Reinhold Platz were more focused on quick and practical implementations, and were helped by the restrictions that the allies placed on the Junkers engine capacity.
The pilots of the time obtained a lot of fresh air. Fresh as in not very warm, since the fumes of the 230 hp Armstrong Siddeley Puma engine were not very far away from the pilot's face. Here is Anthony himself in the cockpit.

Image sources

Wow that looks horrid to try and fly.

Thats one way to keep track of engine operation and performance!

"Hmm.. the inlet valve on #2 cylinder sounds a bit off. I've got my tools down here somewhere..."

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text:
How does an AMRAAM communicate securely with the aircraft that launched it?
From Wikipedia:

It also incorporates a datalink to guide the missile to a point where
  its active radar turns on and makes terminal intercept of the target.
  An inertial reference unit and micro-computer system makes the missile
  less dependent upon the fire-control system of the aircraft.

What is the nature of this "datalink"? What is it called, what protocol does it use?
How is this datalink secure?

Do you have a need to know?
Fortunately, there is a simpler answer to your question: the original AMRAAM does not communicate with the host aircraft. Only the host aircraft communicates data to the missile. This data is transmitted in the form of fixed-format messages, repeated multiple times with redundancy.
Security would fall outside the scope here, except in most general terms. In general terms, when secured, unidirectional datalinks use fixed key symmetric encryption. A missile's downlink isn't necessarily secured, as it's only transmitting the enemy's position for guidance correction.
Newer and more complex missiles that do communicate bidirectionally typically use Link 16. That is a secure encrypted protocol. It's well-documented and you can find a fairly good overview of the protocol on the net.

I'm obviously only asking for public information. I find this interesting because there seems to be quite a big attack surface here from an information security perspective. AMRAAM is quite old, if it uses old ciphers, they may have vulnerabilities, etc.

There's actually an information security SE! Specific information regarding the ciphers is classified due to DOD policy, so no one who knows can tell you whether AMRAAM uses a cipher at all. What they can tell you is that encrypted protocols of that era and that application use Triple-DES with a fixed key. Triple-DES is not especially vulnerable and it's not broken. Key compromise is a different concern. But the midcourse update feature is a very minor factor in the AIM-120's performance.

You would think that, given plane and missile initially being physically connected, they could exchange a one time pad for use that the planes computer generates. Key exchange seems trivial, given that initially there wouldn’t be any trust issues. But I think this is a fascinating question in general, obviously not many people need to know, but that doesn’t make it any less interesting. Lots of interesting questions here, I’d love to read a paper from a pentester that looked at it (obviously I know that such a document will never make it before my eyes. But it’s interesting regardless)

@JustSid I was thinking the same thing. Not just the missile and the aircraft, but all aircraft in the package that will operate together should ideally exchange one time pads before the mission starts --- especially since the missile can receive target updates from AWACS or other friendly aircraft. This is actually a quite complex problem.

I imagine, in the case of a missle and its launching aircraft, you can get away with pretty weak encryption. Since the missle has limited sending power, and it is active only for a short time, the window of opportunity (both temporally and spatially) is quite narrow. Furthermore, the aircraft that's being attacked by the missle needs to have special hacking equippment on board to successfully exploit any weakness. Comms between aircraft is a different kettle of fish, since they are active for years, and probably transmit much further than a simple air-air missle.


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What is the first fighter jet aircraft which carried out a night bombing mission?
Does anyone know in which occasion it was carried out and  the date which it was carried out? Night bombing began in World War I and was widespread during World War II. A number of modern aircraft types are designed primarily for nighttime bombing, but air forces no longer refer to them as night bombers. More common terms today include interdictor and strike fighter, and such aircraft tend to have all-weather, day-and-night capabilities.

Are you asking about bombers? or fighters being used as bombers? or any military aircraft? I'm kind of confused why a fighter jet aircraft would be sent out on night bombing missions. Especially the first jet powered aircraft. There weren't a lot of dual-purpose aircraft in the very beginning of the jet age.

Hard to think of anything like that prior to the B-52 raids on Hanoi.

Welcome to aviation.SE! Are you asking what was the first aircraft to carry out a night bombing mission? Or the first fighter to escort bombers on a night bombing mission? Or the first multirole combat aircraft used for night bombing? If you can make your question a little clearer you may get a better answer.

@JohnK - RAF Canberras were used in night raids during Operation Musketeer, the Anglo-French Suez operation in 1956.

@MichaelHarvey  Brilliant! That sounds like an answer to me.

Canberras weren't 'fighters'.

F80s carried out ground attack sorties in Korea in the early 1950s in the 8th Fighter-Bomber Wing and 49th Fighter-Bomber Group, I haven't found a record of any night missions but there may well have been - https://en.wikipedia.org/wiki/Lockheed_P-80_Shooting_Star#Korean_War


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Are there any airworthy or transportable A-7 Corsair 2 aircraft as of October 2021?
I'm assisting in organizing an Aviation Open House, and we are searching for an A-7 Corsair 2.  The only examples we can find are static ground displays, none of which can be easily transported.  As of today, are there any air-worthy A-7 Corsair 2s in existence?

Have you tried searching the FAA registry for Manufacturer Name = Vought and Model Name = A7 ?

Based on the internet searching I did it doesn't look very promising.
List of surviving A-7 Corsair IIs:
https://en.wikipedia.org/wiki/List_of_surviving_LTV_A-7_Corsair_IIs
It looks like Greece flew the last ones, retiring the last one in October 2014. I have no way to see what they did with them after retirement.
https://theaviationist.com/2014/10/27/haf-retirement-a-7/
I don't know how current the info is at this site:
https://www.planelogger.com/Aircraft/ConstructionList/A-7
It lists ZERO A-7s in the Hellenic Air Force fleet (not even showing any as being stored):
https://www.planelogger.com/Airline/Fleet/Hellenic_Air_Force/172406
I just searched David Monthan AFB in Tuscon on Google Maps to see if there are any A-7s  because of this photo:  https://www.airliners.net/photo/USA-Navy/LTV-A-7E-Corsair-II/2446637
and do not see any A-7s stored outside there.
That's all I can figure out on the internet. Hopefully someone who has more specific knowledge will come along (I'm not a warbird expert by any means).

I know there's a mostly functional one that can still fly in the Tulsa Tech Riverside campus in the state of Oklahoma, the campus is primarily for teaching students how to preform maintenance on an aircraft, and from what I remember, the instructors preform the maintenance on that aircraft, and occasionally start up it's engine. I'm not 100% sure it would be able to fly on a moments notice, but I'm pretty sure it would just need a minor bit of work and it would be good to go for flight from what the instructors have told me.

Can you give us a link to this?


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XFOIL - Modify the gap at the trailing edge of the airfoil
I'm working on inviscid analysis flow in XFOIL. The trailing edge of my airfoil is too sharp, therefore leading to non-convergence of the analysis. Modifying the trailing edge thickness may do the trick. How to adjust the trailing edge thickness in XFOIL? I read online that you have to use the command GDES > TGAP. However, I couldn't find anything regarding of what to input the command. The documentation was not helpful either. I would like to change the thickness to 20% of the chord length.

Where do those 20% come from? Is that a typo, or is it a root airfoil for a wind turbine? The latter can be truncated for transport reasons.

XFOIL is happy with a sharp trailing edge and will tolerate a gap of a few percent. However, 20% is way beyond of what it is meant to receive as input, so I would not trust the result.

How to adjust the trailing edge thickness in XFOIL?

From the starting point you should first load an airfoil (for my example I used the command NACA 2412 to get a simple NACA four-digit airfoil).
Then I changed into the geometry design subset with gdes and continued like this:
.GDES   c>  tgap

 Current gap =  0.00252

Enter new gap   r>  0.02

Enter blending distance/c (0..1)   r>  0.1
 Max thickness =     0.120041  at x =   0.297
 Max camber    =     0.019999  at x =   0.403

.GDES   c>

The result is an overlaid plot of the old and the new geometry, like this:

To set a sharp trailing edge, set tgap to zero. For this example, I continued into the operating subset and calculated the inviscid flow at an AoA of 4°:
.GDES   c>  tgap

 Current gap =  0.02000

Enter new gap   r>  0.0

Enter blending distance/c (0..1)   r>  0.1
 Max thickness =     0.120031  at x =   0.297
 Max camber    =     0.019999  at x =   0.403

.GDES   c>

 Buffer airfoil is not identical to current airfoil

 XFOIL   c>  pane

 Sharp trailing edge

 Paneling parameters used...
   Number of panel nodes       160
   Panel bunching parameter    1.000
   TE/LE panel density ratio   0.150
   Refined-area/LE panel density ratio    0.200
   Top    side refined area x/c limits  1.000 1.000
   Bottom side refined area x/c limits  1.000 1.000

 XFOIL   c>  oper

.OPERi   c>  alfa 4.0
 Calculating unit vorticity distributions ...

.OPERi   c>


text:
MIG 29 Engine - how to interpret these values?
I was wondering how powerful the engines of the MIG 29 are. On Wikipedia I found some specifications for it.

How is the mass flow rate calculated? I know mass flow rate = density * area * velocity.
Is this the mass flow rate for max thrust?

Is it possible to determine the thrust / mass flow rate in the operating point (so I mean where you normally fly with, for which the engine is designed for high life cycle)?

It's not stated in the figures listed but I think it's safe to assume the mass flow rate is for maximum thrust at sea level, when the engine/aircraft is stationary. This removes most variables with engine installation/inlet geometry. Mass flow rate widely varies with airspeed, Mach and altitude, so your second point will be quite hard to answer.

Generally, data found is sea level static take-off power unless stated otherwise.
Mass flow cannot be measured easily, so indeed, mass flow is calculated, but not how you expressed.
Dealing with gas turbine performance data, the best tool to calculate the mass flow is using a gas turbine simulation tool (the mass flow is usually not given). If there is more data available (e.g. thrust, specific fuel consumption, turbine entry temperature), these can be used to calculate the mass flow. Gas turbine performance engineers furthermore take into account the technology level, the overall compression ratio to forecast turbine cooling flows and apply turbo machinery (fan, compressor and turbines) off-design characteristics maps so that they can make calculations of other performance points than the take-off power.
The flow in the design/reference point (the point where you have the most data available, usually take-off power) is found iteratively when tuning the model. The mass flow for any other operating point is calculated by the model based on the input parameters (ambient/flight and conditions  power setting).


text:
Why complex landing gears are used instead of simple, reliable and light weight muscle wire or shape memory alloys?
Muscle wires actuators are really simple, have a high power density and are reliable. Landing gears on other hand are used only twice per flight while taking off and landing (if everything goes well though). During flight these machines are not useful and there are numerous gear up landings because of landing gear failures.
What is so complex in landing gears?
Why are electric motors/hydraulic actuators used instead of muscle wire actuators in landing gear which are simple, light weight, reliable and cheap?
(If there are electric power requirements, that can be achieved by supercapacitors.)

You may provide some links or reading about muscle wire actuator as I'm not sure how familiar is the aviation.SE community with this kind of actuator and how it is operated. Moreover, you may restrict your question to one kind of aircraft as many GA aircrafts are equipped with fixed landing gear

my preferred tech is 100% reliable > all others are not

"There are numerous gear up landings". Source? For an airliner, a gear up liner usually means to write the aircraft off. We don't see many of those, by any means.

"Why are electric motors/hydraulic actuators used instead of muscle wire actuators in" X could be asked about any field that uses electric motors or hydraulic actuators for any limited range movement - from small printers to earth moving equipment. If muscle wire are ever going to replace current devices in a wide range of fields, I don't expect planes to be among the first instances.

Can you provide any large-scale industrial application of muscle wires at all? To me it appears that muscle wire technology is pretty much useless outside of toys and experiments so far.

Some evidence of the asserted reliability???

Shape-memory alloys change their shape with temperature, with control usually achieved with electric heating in one direction, and back with convective heat transfer to the environment. There is one specific temperature where the change in shape occurs. There are two huge problems with this:

The operating temperature range for gear actuators is huge. It may easily be below -40°C on a cold arctic night, but on a hot tropical day the sun can easily heat the structure to upward of +60°C. That means the transformation temperature must be pretty high, but that means a lot of energy has to be put to heating it, especially if the ambient temperature happens to be low on the day.
Note that the actuators are not very energy efficient. A super-capacitor can give you high peak power if you need that, but will not help at all with the total energy. The work the actuator has to do is significant, so the efficiency matters.
Worse, a shape-memory alloy actuator requires constant power to remain at one end of the motion range and spontaneously returns to the other. However, gear must be stable in both end positions without requiring energy, so there would still have to be uplocks and separate actuator for extension or retraction (or maybe extension would be gravity-only, but then you have to ensure it gets reliably locked). Not really simple anymore.

I also don't think they actually scale to the required forces and displacements well. A 2 mm wire bends quite easily, 2 cm rod, not so much. The thicker will of course need more force to bend—which is OK—but it will also break much sooner, because the difference in strain is bigger.

Point 2 might be removed if engine waste-heat is used.

@leftaroundabout Using engine heat at the gear seems to fly in the face of 'light, reliable' mechanisms like the question asker seems to be seeking.

@Saiboogu for a jet plane – definitely. For a GA craft with a single piston engine, this wouldn't require much more than a bit of clever exhaust piping though.

@leftaroundabout So if the engine fails, you wouldn't be able to extend landing gear? That's obviously a bad idea.

@StephenS actually, if the engine fails the landing gear would automatically extend.

@leftaroundabout That could equally be as bad. As a pilot, I want the gear out when I tell it to go out. Just because my engine dies doesn't mean I want a huge drag-inducing thing hanging out of my plane. Let me pick the landing spot, glide there, and extend the gear when I'm assured to make the landing spot.

@leftaroundabout Let's hope the aircraft type never has an all-engines-out landing, if the landing gear requires engine heat :)

@leftaroundabout It's worse than what RonBeyer said. Uncommanded deployment of the landing gear at high airspeed could permanently remove it from the plane. Or worse than that, also remove a few bits of wing from the plane...

@alephzero why so unimaginative? Of course you wouldn't install such a system on anything that would ever fly over water, or require more than a field for landing, or be able to go high speeds. This would clearly be a specialised design for extremely light craft, e.g. super slow crop dusters or city air taxis.

The weight/complexity of modern landing gear is not in the extension mechanism, that is relatively simple (a hydraulic cylinder typically).
The complexity of the gear comes from the robustness of the design.

Source: Flyertalk
(Not the greatest picture), but the retraction mechanism there is the simplest part of the gear. The complex part is the linkages/hydraulic dampeners (shock absorbers), brake systems, etc. The structure needs to be there because you have a (in the case of an A380) a 1,234,000lb (600 tons) aircraft slamming into the pavement at about 300 feet/min.
There needs to be a hydraulic system for the brakes, so they already have hydraulic systems there. The other nice thing about hydraulics is that they can be extended manually in the event of an electrical failure.
So what would you really gain by introducing a complex system like muscle wires, given the additional failure modes? For example lets say your gear bay is really cold from your trans-atlantic flight at 45,000 feet and your electrical system fails? Is your Ram-Air turbine and batteries going to be able to supply enough current to keep the system extended, or retracted if need be? You don't want your gear hanging out when you are trying to make the airport in a glide.
Hydraulics are used because they are proven cheap, reliable, robust, and easy to work on. Many aircraft systems are hydraulic (including control surfaces) for exactly that reason, as well as muscle wire only being around for a short time.
That isn't to say that shape-memory alloys don't have an application in the aviation world. Many aviation companies are looking at using SMA's in applications such as engine control and wing shape shifting applications, but they are a long way from being used in practice.


text:
How does this fuel pump work?
This question is about the fuel pump controlled by the hydro-mechanical fuel control unit.

This is part of a fuel system for a turboprop engine, described on this page which actually comes from a Rolls-Royce book. Full picture for convenience.

I've been staring in vain at this picture for some time, with the hope that something will suddenly become clear:

Source
The pump is supposed to receive low pressure fuel and deliver high pressure fuel at a rate modulated by the FCU using fuel in the purple pipe. The engine speed governor also modulates the pump output.

It seems to be a piston pump, pistons being moved in translation by the rotating plate on the left side, but I'm not even sure, and I don't understand how this increases pressure.
It seems the governor is not rotating... and I don't get the role of the green governor pressure fuel.

Can you help me understanding how it works?
(I've posted all the details I could gather as an "answer").

Does the description on the "Read More" link offer the info you are looking for?

@RonBeyer: Unfortunately not.

Sometimes diagrams can be mystifying. This is a video of a variable displacement hydraulic pump of similar principle being torn apart (disassembled would be too gentle a term) by a foul-mouthed Canadian who gives an excellent detailed & clear description of its construction & operation. Language warning for those not used to frontier engineering.

Which will prepare you for this very polite Canadian who shows an actual aircraft fuel pump. (around 7:30)

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text:
Where does turbine vane and blade cooling air come from?
Some large jet engines have a high combustion temperature to increase maximum thrust. First vanes and blades of the turbine must be cooled to prevent them from melting. One method is to circulate cold air from the compressor inside the element and/or create a coating of air on the element faces using channels and holes.

Trent turbine HP blade with cooling channels, source
How is air from the compressor channeled to the vanes and blades? How is cooling air prevented to escape to the space between rotor disks instead of going to the cooling channels, under turbine gases pressure?
What approximate fraction of core power is used for such cooling?

I don't think you need a high pressure difference. By approximation, the combustion chamber is constant pressure (subsonic flow!) and the faster moving fluid due to combustion creates a lower dynamic pressure (Bernoulli). I'm not sure about the specifics of this system though

Basically, the 'cooling' air is obtained from the diffuser and/or combustion liner (it will differ in different engines) and is routed both externally and internally (i.e through the casing and spindle) to the nozzle and rotors. Though the details differ from engine to engine, the basics are similar, for example to the NASA Energy Efficient Engine. From NASA CR- 167955 High Pressure Turbine Test Hardware Detailed Design Report:

The Stage 1 nozzle is cooled by air extracted from the inner and
  outer combustion-liner cavities. The vane leading-edge cavity is fed from the inner flowpath, and the aft cavity is fed from the outer flowpath ...
The Stage 1 rotor is cooled by air extracted at the diffuser mean line.

The image below shows the schematic of the airflow inside the engine used for cooling.

Engine cooling air supply; image from NASA CR- 167955 High Pressure Turbine Test Hardware Detailed Design Report
Latter stages are cooled with air derived from higher compressor stages. For example, in this engine,

The Stage I shroud is cooled with air supplied from the compressor dis-
  charge.

and

The Stage 2 nozzle cooling air is extracted from the compressor at the
  exit of the seventh-stage stator.

As can be seen in the figure, the cooling air is prevented from escaping back using a number of seals.

Engine seals; image from NASA CR- 167955 High Pressure Turbine Test Hardware Detailed Design Report

Interesting linked document, thanks.

To build upon a few points in Peter's answer:
This diagram will probably help visualize it (I copied from this question How is the central hub / shaft casing of a two-spool jet engine assembled?):

The seal between the disks is just a labyrinth seal.  Those chambers, however, are pressurized (by similar mechanisms as the cooling air was delivered to the blade). So there is a tiny positive flow of air out of that chambers, which prevents the hot combustion gases from flowing in.  Refering to the diagram, the space in front of the 1st stage disk is pressurized from air coming from the stg 10 compressor through the series of passages colored in orange.  The space between HPT stage 1 and stage 2 is pressurized by air the comes along the inner diameter of the HP rotor, from an impeller between stage 7 & 8 compressor (in blue), and the space aft of the HPT stage 2 is pressurized from air the comes from the stage 4 compressor (by means of pipes on the outside of the engine, light blue color).
Pretty much every cavity/chamber inside the engine is pressurized somehow, to keep air from going in undesired directions.
I don't know exactly how much air is used for cooling. It's on the order of 1% (i.e. I know it's less than 10%, but certainly more than 0.1%)

Nice answer, the diagram is helpful.

Since the pressure of cooling air must be higher than the local pressure in the turbine, this air needs to come from the diffusor area, which is where the highest pressure in a gas turbine is found (see here for details). Normally, it flows through the hollow high pressure spindle so it arrives at the root of the turbine blades from where it finds its way to the surface of the blade. Centrifugal forces help to push the air through the narrow holes, so in effect the turbine blades suck the air in at their root. The seal between the disks is (to my knowledge - I am on shaky ground here) nothing better than a labyrinth seal.

Between the discs there's isn't any special seal. In some older engines, the surfaces are made flat (typically 50 microns) and bolted together with anything from 50-150 bolts depending on the engine. In newer engines they're welded together. Labyrinth seals are used between objects rotating at different speeds (I.e. Between shafts)

@Notts90: Welding will make maintenance impossible, so the whole turbine section must be replaced in one go. Is that really so?

I know at least one major manufacturer welds some of the compressor discs to make a set of 2-3 stages in one of the newer engines. I don't think its uncommon to have 2-3 stages permanently joined together. A different manufacturer actually machines 2 stages inc. blades from a solid piece of metal and that's an old engine.

@Notts90: Ah, in a compressor this makes more sense. The fully machined stages are called blisks, as in bladed disks.

They are indeed!

Some manufacturers of stationary gas turbines do use welded rotors also on the turbine side. Blades are different parts (also different material) and get assembled axially, with heat shields in between, which form the counter part of the turbine vanes. All the gaps between the components (heat shields, blade platforms) are sealed


text:
Could fly-by-wire protect against a takeoff tail strike?
If the takeoff weight is underestimated, rotation will be done too early and may result in a case of tail strike.
One example of such mistake is a 747-400 (F-HLOV) in 2006 (case 8 in this study). The crew entered ZFW for TOW (100 tons lower), VR was underestimated by 32 kts.
Boeing documentation mentions every aircraft can be subject to tail strike, at different degrees. Airbus documentation for tail strike prevention doesn't mention any specific protection from FBW.

(source: The Aviation Herald.)
UA B744 tail strike on Sydney takeoff (photo may not be the actual May 7th 2010 T/O).

I wonder why such basic mistake may be done. Weight could be sensed in some way by the instrumentation, and crew entered values should be challenged if really different.
To prevent the crew from being forced (or tempted) to rotate the aircraft beyond the maximum safe angle, isn't that possible to monitor the T/O progress and in case of low performances alert the crew, and provide an opportunity to reject the T/O.
Airbus FWB embeds many security systems (compared to Boeing), this is surprising that they cannot prevent such T/O tail strikes. I'm sure there are good reasons, just asking to understand the difficulty.

Edit: Based on the answer from Marky Mark, who references the EK407 case, I checked a study published in the aftermath of EK407 tail strike: Take-off performance calculation and entry errors: A global perspective and noted this recommendation for 'TOPMS':

While the above recommendation does not preclude data entry and
  calculation errors relating to take-off performance parameters from
  occurring, Transport Canada (Department of Transport) agreed that a
  take-off performance monitoring system(s) (TOPMS) would provide a
  significant safety benefit. However, before regulatory authorities
  establish a requirement for the fitment of TOPMS, a certified system
  would need to be developed (Transport Canada, 2010).
Basically, a TOPMS, which assists pilots in determining whether to
  continue or reject the takeoff, can be defined as (Brown & Abbasi,
  2009, p. 7).

Are TOPMS now in place?

I won't post this as an answer because I don't have time to elaborate: but essentially it's possible. Remember though that in the case you mentioned, the crew misjudged the weight: if the aircraft was working off the same data, it would make the same mistake. We could add the ability for the aircraft to weigh itself, but that wouldn't need to be part of the fly by wire, it would also have solved the problem in the example mentioned. Essentially it adds weight and cost for something which could be handled by better procedures.

Adding to Jons answer, fly by wire sure is a great thing, but the more you automate it the less control the pilots really have over the airplane. Imagine yourself in a car where you are about to crash into another car and want to leave the lane  to go into the ditch, but your car won't let you because it is automated and wants to stay in the lane... Me as I pilot would like to make the decisions in an airplane, and (though I can't think of am example but also generally speaking) maybe sometimes you'd prefer a tailstrike rather them going trough the fence at the end of the runway...

@Maverick283: Agreed, though there is an alternative to all-manual and all-automated, as stated in the TOPMS quote (assist pilots in determining whether to continue or reject the takeoff)

@mins Jupp, didn't mean to fight that or anything. While this is mainly about answering your question, there will be others looking this up eventually. I personally find it best to give inputs so that other people get as much out of a question as possible!

@Maverick283. I'm not sure to fully understand (I'm not a native English speaker). I'm looking for an answer that would make a synthesis of all the sensible posts made (incl. yours). If you are in the mood for this... :-). Overall I made my mind this way: Mistakes on weight could be prevented (sensor for weight), could be detected before V1 (by monitoring actual acceleration), but it is not desirable to prevent the pilot form pitching and creating a tailstricke (because risking TS might be the best option compared to RW overrun).

There is a simple solution to preventing tail strikes that was employed by the first 747 carrier I flew for. Their takeoff procedure called for an initial rotation to 10 degrees nose up and then holding that attitude until liftoff. The downside of that procedure was that you used more runway than you would have otherwise, and, of course, it depended on the pilot to actually do that.

Boeing has removed the tail skid on the latest B777-300's because they have never had a tail strike and an updated version of FBW will virtually guarantee it going forward. Removing the tailskid saves about 300 lbs.
More info:
Aviation Week Article

Boeing has also decided to remove the tail skid from the 777-300ER as a weight and drag reduction improvement after developing new flight control software to protect the tail during abused takeoffs and landings. “We redesigned the flight control system to enable pilots to fly like normal and give them full elevator authority, so they can control the tail down to the ground without touching it. The system precludes the aircraft from contacting the tail,” Schneider says. Although Boeing originally developed the baseline electronic tail skid feature to prevent this from occurring on the -300ER, the “old system allowed contact, and to be able to handle those loads we had a lot of structure in the airplane to transfer them through the tailskid up through the aft body into the fuselage,” he adds. “So there are hundreds of pounds in the structure, and to be able to take all that out with the enhanced tail strike-protection system is a nice improvement.”
The change was implemented on the line in November and will be offered as a retrofit through a service bulletin. “With a retrofit, you can’t save so much weight because the structure is already in the fuselage, but the drag and maintenance savings is still a nice benefit,” says Schneider. “It is also one of things customers are most interested in retrofitting off the aircraft."

Tail Strike Protection (TSP) logic has been incorporated in the 777 FBW since the -300ER became available. It is available on the 777-200LR as well (despite shorter body); I believe all other models also have it now through the PFCs (200ER, F). The newer -300ER aircraft have the TSP logic ENHANCED and with it came the removal of the tail skid, which saves 323 pounds (145kg) of aircraft weight besides maintenance costs (it's a hydraulic actuated skid).

You're saying they built over rotation protection into the EGPWS or that 773 pilots are good enough that they never over rotate and have a tail strike? (there's a :) for the 2nd half of that question)

It is all in the software. See my edit above.

Thanks for the additional info. That really was intended to be a bit of a tongue-in-cheek comment, but your answer is even better now.

It is not the pilots.  I belive it is an updated FBW. ( not EGPWS as I said earlier)

On Airbus aircraft, the guidance law provides attitude protection on take-off which should prevent a tail strike. However, this is not fool-proof and it remains up to the PF to maintain the correct nose-up angle.

Thanks for the reference to EK407. Based on ATSB report, it is very similar to Corsair: -100 tons TOW error in FMGS, early rotation. No real protection (e.g. low acceleration alert prior to V1), only the FD cue to achieve T/O profile speed. The good news is the related release of Take-off performance calculation and entry errors: A global perspective, a study of 20-year tail strikes (including F-HLOV).

Sorry but there is a mistake in Marky Mark's answer. The Airbus does possess an attitude protection but it doesn't protect against overrotation. If you are doing a standard flaps 1 takeoff, the protection steps in at 30 degree nose up pitch(!). As stated in the 320 typerating website link, the normal rotation rate of 3 degrees per second needs to be adhered to! In other words, the protection is(!) fool-proof but is far from active at a takeoff.


text:
What does Ryanair do without Sick Bags?
Sorry for the slightly strange question, but Ryanair (Europe's Largest Low Cost Carrier) decided that seat back pockets took too much effort and got rid of them, and it appears the sick bags in the process.

(source: poloconghaile.com)
So the question comes up...
What happens when Ryanair flights encounter turbulence? I can hardly imagine the Flight Attendants handing out bags ad-hoc when it's sufficiently shaky to cause motion sickness among the passengers.

Well, what they save in sic sacs they'll easily spend on cabin cleaning. Next step: Hard plastic / vinyl cabins that can be hosed out after every flight? (Also, this video is obligatory)

BYOB - Bring your own bags

@flyingfisch O'Leary has been accused of hinting at a lot of things. I don't think they'll charge for a sick bag as it's more expensive to clean up the aircraft (and I don't think they can charge you for that, at least not if they didn't give you any choice). RyanAir is about cutting costs at every corner, not about screwing people over (although sometimes it does amount to that), but given two options, I'm pretty sure they'll certainly pick the cheaper one, which in this case means giving you a bag if you ask for it.

@voretaq7: they don't clean it, that saves more money. By going green and letting natural decomposition take its toll, they are helping to support many animals on Earth and improve their chances of living.

LOL @Qantas94Heavy! Very environmentally friendly of them!

Well how many people get airsick during each flight and how many of those really vomit? And of those, how many actually use the provided sick bags? I'm not familiar with getting air sick but I'm sure there are not a lot people really using these bags. There is also no legal requirement for having sick bags. It is an item for 'additional passenger comfort'. Ryanair probably thought this way and so they saved some more money.
They also don't hand out sick bags when they are encountering chops but I'm sure they will have one for you if you ask them.

most of the time I've seen someone nearly-sick, they call the attendant proactively to get relief with water or other creature-comforts that they know can calm their symptoms.

Maybe the idea is that they expect frequent fliers to get in the habit of carrying sick sacks to hand out to the passengers seated adjacent to them, to save on dry cleaning bills etc.

@quietflyer - Damnit. Just one more thing that I have to put in my carry on. I actually used to do that in the military for new paratroopers. The alternative was enduring the mix of smells in the back of a military aircraft. Since, the alternative to my sick sack was to have the private throw up in their k-pots (which they had to wear for the jump), or inside their t-shirts.


text:
Is airflow ever laminar in the boundary layer, near the wing leading edge?
I often see images of the boundary layer over a wing indicating the airflow is laminar over the first portion of the wing:

Transition from laminar to turbulent flow over the wing. Left: source, right: source
I was also told this cannot happen in a full-scale aircraft, as the Reynolds number involved are not compatible with laminar flow.
Would these pictures be misleading?

Is the flow laminar or not before the boundary layer is separated?
If so, what are the conditions that make a flow transitionning from laminar to turbulent?

Look here for details on how transition works.

It may help to picture the wing at its proper AOA.  It seems the "laminar" theory of the 1940s has evolved into "supercritical" design today.  It may have been that the P-51s lift vector was tilted a bit forward with the "laminar" wing, but it worked, the reduced net drag gave it the ability to fly a long way.

In still air every boundary layer starts laminar. How soon it transitions to a turbulent boundary layer depends on:

the local Reynolds number,
the pressure gradient,
wing sweep and
disturbances like bugs, rivet heads, waviness or turbulators.

Flat plate flow (without pressure changes) normally transitions at a Reynolds number of around 400,000. If the flow is accelerated, all speeds in flow direction increase while cross flow will not be affected, so a laminar boundary layer in accelerating flow is stabilized. On modern gliders the lower surface is laminar in excess of 80% chord at higher angles of attack, which can correspond to a Reynolds number of 5,000,000 or more when transition eventually occurs.
On the other hand, a pressure rise in flow direction corresponds to a deceleration in flow direction, so any movements perpendicular to the flow direction will grow relative to the flow speed, and as a consequence the turbulent transition occurs rather quickly. Upper side flow past the suction peak near the leading edge is a prime candidate for transition, and that is what causes flow around the "traditional airfoil" to become turbulent earlier. The graph in your question is misleading because the lower side flow of the traditional airfoil should be as laminar as that of the P-51 airfoil if the surface smoothness of both is comparable.
How laminar was the P-51 wing?
At the flight speed of the P-51 very little laminar flow was left; the full effect of laminar airfoils can only be exploited at Reynolds numbers below 5,000,000. At higher Reynolds numbers it needs progressively steeper gradients to keep the boundary layer laminar, such that the range of angles of attack where a long laminar boundary layer is possible on both sides of an airfoil (the laminar bucket) gets smaller and smaller.
The "rooftop" distribution of the 6-digit NACA airfoils did help, though, because it gives them a higher critical Mach number than the "peaky" distributions of earlier airfoils. The suction peak near the nose of older airfoils will lead to local supersonic flow at a lower flight Mach number, and increased drag from the shocks which would follow. Most important for its low drag, however, was the very smooth wing surface of the P-51 with no gaps ahead of the spar. See this rec.aviation.military post for details.
Measurements on the P-63
The Bell P-63 Kingcobra used early laminar airfoils, the NACA 66(215)-116 at the root and the NACA 66(215)-21 at the tip. British testing of its wing suggests that the build quality of the period's metal wings was insufficient to maintain laminar flow. From the linked Wikipedia article:

The RAE first tested it in an "as delivered" configuration. The wing airfoil was designed to support laminar flow to 60% of chord. In the "as delivered" configuration, a profile drag was measured which was representative of the wing section with boundary layer transition at the leading edge (0% laminar flow). Reducing the surface roughness reduced the drag at low lift coefficients to a level representative of laminar flow to 35% of chord. Measurements were made of the surface waviness. This showed peak wave amplitudes, above the mean, of approximately 0.011 inches (0.28 mm) over a two-inch (5.1 cm) span. The standard waviness criteria shows the critical wave height to be 0.0053 inches (0.13 mm) for this application. To reduce the waviness, RAE personnel stripped the wing to bare metal. The wing was then sprayed with two coats of primer paint and a coat of paint type filler. After the paint was dry, it was sanded in a chordwise direction, using sanding blocks, whose curvature matched the local surface curvature. This was repeated several times. Surface waviness was then measured and found to be no more than 0.005 inches (0.13 mm).
In flight, this configuration was found to have a profile drag representative of boundary layer transition at 60% of chord. This gave researchers an idea of what level of wing surface quality was required to actually get the benefits of laminar flow airfoils.

Influence of Sweep
Wing sweep will also make it hard to maintain laminar flow. As explained here, on a swept wing only the speed component perpendicular to the wing will be affected by it, so the accelerating flow past the stagnation point will curve inwards on a sweptback wing. At the same time, viscosity will slow down the flow near the wing skin. The consequence is a twist in the speed distribution over the boundary layer, which destabilizes the laminar flow and leads to early transition.
A C-172 with its four-digit NACA airfoil has the peaky upper surface which will trip the boundary layer very early on the upper surface. On the lower surface laminar flow will last a little longer but will be destabilised by gaps in the surface, so most of the flow on the C-172 is turbulent. On an airliner the Reynolds number is in the tens of millions, so transition will be very early and very little laminar fraction remains. This is mostly found near unswept leading edges like the engine nacelles. Only with advanced technologies like boundary layer suction will it be conceivable that a larger part of an airliner wing can be kept laminar.
Transition and Separation
Laminar separation sometimes happens when the flow separates shortly after negotiating the nose, like on a five-digit NACA airfoil or an undimpled golf ball. This leads to an abrupt stall and should best be avoided. Normally, the boundary layer transitions into the turbulent state and stays attached until the turbulent boundary layer separates, either at the trailing edge, or progressively further ahead of it when the airfoil stalls.
Sometimes, transition happens in a laminar separation bubble. The decelerating flow past the suction peak is slowed down by friction near the surface, and both effects combine to bring the flow to a standstill at some point. The boundary layer thickens, such that the pressure rise is momentarily suspended, and speed oscillations in the boundary layer get amplified such that cross flows become more intense, mixing outer and inner parts of the boundary layer. The speed profile becomes fuller and the flow near the wall picks up speed again, such that the separation disappears and pressure rise resumes.
Below I plotted the XFOIL results for the pressure distribution around the HQ-17 at Re = 1 Mio (the HQ-17 is used on the ASW-22 Open Class glider, for example). The dashed, black lines show the inviscid pressure while the colored lines show the viscous flow results. On both sides you will see a kink in the colored lines - this is where the laminar separation bubble is.

When the flow separates, the pressure line becomes horizontal. After transition it jumps back down near the inviscid line, which shows how much steeper the pressure gradient is that a turbulent boundary layer will tolerate. Reattachment is complete when the steep pressure rise has brought the local pressure back near the inviscid level. Note in the airfoil plot at the bottom that the boundary layer thickness peaks where the separation bubbles are.
Yes, the boundary layer is laminar before and into the separation here. This phenomenon occurs at the scale of model aircraft, gliders and small GA aircraft (100,000 < Re < 5,000,000) but is absent at higher Reynolds numbers because then the transition happens before the laminar flow separates.

Reynolds number is basically the ratio between the viscous and inertial forces acting on the system. At low Reynolds numbers, the viscous force is dominant compared to the inertial force and it is the other way at higher Reynolds numbers.
The effect of the viscosity can be considered analogous to the dampers in a car suspension. If the damping is good, the suspension eats up any bumps on the road and the ride is smooth. In case of fluid flow, if the viscous term is dominant (i.e. if the Reynolds number is low), small disturbances in the velocity field, caused by perhaps surface roughness or vibrations, are not allowed to grow and the flow is smooth; however, as the Reynolds number increases, the viscous damping reduces and the small disturbances in the fluid field can grow and the flow becomes erratic i.e. turbulent. The flow can also become unstable due to resonance.
It's important to note that the turbulent flow (which is found over most of the commercial aircraft wings) is different from separated flow. Flow over most of the wing is turbulent, but attached. The Reynolds number of large commercial aircraft increases over the wing at the rate of over a million per foot. In these cases, the flow is rarely, if ever, laminar. In fact, Boeing admits:

Current Boeing commercial airplanes are not expected to have significant regions of laminar flow, with the exception of the 787 nacelles near the inlet lip, as expressly designed. There may be very limited regions of laminar flow near the leading edges of wings, especially lesser swept wings such as used on the 737 and 757, and blended winglets.


text:
Why is the airflow in airline cabins downwards?
Air is distributed from overhead outlets and flows downwards towards the floor. Since the exhaled air is hotter than the air around, and tends to concentrate towards the ceiling, wouldn't it make more sense to pump air from the floor upwards towards the ceiling?

"Since the exhaled air is hotter than the air around, and tends to concentrate towards the ceiling" -> You can turn that argument around: if the air conditioning outlets would be near the floor, the cold air would just concentrate there.

Temperature stratification is a vexing problem on lots of airliners.  The big challenge is you need a distribution system that is effective when cooling (on the ground and at low altitude when the weather is hot) and heating (as soon as the outside temperature drops to the point where there is a net heat loss that can't be made up by passenger body heat, the ECS will be producing inflow that is warmer than ambient).  At cruise where the  outside temperature is generally -10 to -40C, you are in heating mode, where inflow temperature is above ambient.
The objective is to get a circulation going (air falling along the side walls, and rising at the center aisle) to keep the air mixed and prevent temperature stratification in the small space, with the added factor of a large heat emission from the passengers to cope with. Distribution systems strive to do this.

So you would think that most of the time, you want the air coming in at the floor so there is a natural convective circulation favoring heat from below (most of the time incoming air is warmer than ambient).  Problem is, you have the heat of the passengers, which is significant, rising into the ceiling.  You generally want to measure temperature for regulation purposes at somewhere around passenger head level, and if the heat comes in at the floor, there is a tendency for heavy stratification as the sensor is remote from the incoming heat source and it creates a very long response loop (sense, adjust, wait for the result, adjust again, etc.).
The CRJ200 suffers badly from this problem because the airflow is from Dado panels along the sides at the floor like a home heating system (a design carryover from the Challenger corporate jet, which had different distribution requirements).  The temperature sensor is up by the overhead bin.  When the sensor called for more cooling, or even just less heat, the Environmental Control System would dump in all kinds of cold air, which didn't want to rise being colder than ambient, so it would have to, in effect, fill the bottom of the cabin space with cold air (or really, less hot air) until the "coldness" reached the sensor up at the bin.  Anyone who's ridden in the CRJ200 knows well the cold feet problem with that airplane, even at altitude where it's -30 outside.  What helped a lot was moving the sensor down near the floor to the inflow region to reduce the response loop, and a modification was created to do that which was quite effective, but few airlines bothered with it and having the sensor near the floor is generally suboptimal from a design perspective.
On the later CRJ versions they went with the more common airliner arrangement.  Air is injected up at the overhead bins somewhere and most of it is shot at the sidewall panels, to get a waterfall sort of flow down the sidewall panels, inboard along the floor and up the center aisle(s) (some air may also be injected into the space between the bins to prevent stagnation there).  This works reasonably well even when in heating mode where the air being injected down is warmer than ambient, and allows the temperature sensor to be in a good location with a reasonably short response loop, so variations of this type of distribution system are the most common.

An excellent example of why "air in at the top" is right and what happens when the designers get it wrong!

Ha, this makes a lot of sense.  I think every commercial flight to my regional airport is on a CRJ200, and they're always fighting with the heating/cooling.

Sometimes the pilots just give up on the auto system and run the system in manual, where one them has to monitor it and tweak the temperature switches from time to time. Moving the sensor to a box under one of the seats fixed about 80% of the problem, but nobody wanted to pay for the expensive field mod.  One airline created their own version of the floor sensor as a field mod at a fraction of the cost, and as far as they were concerned it completely fixed the problem for them.

Not at all. Cold air tends to go low and hot air high. If the cooled air flow were pumped in from the floor, then what we would have would be customers with cold feet and hot heads, making an already miserable experience even worse.
By putting the nozzles above the head, the airline is directing the cooled air on the hottest parts of the masses of bodies crammed in the cattle-car.... err I mean airliner.
I perform much the same at my home. During the winter months when my conditioned air is pumping in heat, I adjust the baffles so that most of the heat comes in on the first floor and some on the second and a bit on the third floor. The heated air naturally rises up to the third floor.
Conversely, during the summer months, I re-adjust the baffles so that the majority of the cooled air comes into the third floor, some on the second and a bit on the first. The cooler air flows down.
Pumping the cooled air into the first floor (or floor of the cabin of the airliner) would necessitate cooling the mass of air enough so that the higher, and warmer, air mass is (eventually) also cooled. By that time, everyone's toes are cold and they are complaining.

The OP is asking why there is air distributed from above. He states since exhaled air is hot it congregates in the upper area of the cabin. I say if the cooled air were pumped in from the floor, the passengers wouldn't feel it as quickly, as it would have to cool the entire cabin from the ground up, versus pumping it in from above the pax's heads, they feel the cooling effects immediately on the parts of their bodies that are warmest, and the cool air will continue to flow downwards. He asks why it isn't pumped in from below and I answer that question.

Also, it would be very hard for passengers to adjust their personal airflow if it came from under the seats.

@ToddWilcox the gasper system (those little ball vents) is separate from the main distribution system. The air coming from the  gaspers is usually recirculated cabin air.  The inflow on an overhead distribution system is coming from slots at the overhead bins that shoot air out that you have no control over.

As an addition to the other answers I want to mention that feet tend to stink - especially when lots of passengers take their shoes off. You don't want that stink to rise from the bottom to the top, where people can actually smell it, so you have the fresh air comming from the top and the stinky air being sucked away at the bottom.

This probably isn't the reason, but it's definitely a beneficial side effect!


text:
Why are we still using audio to communicate between ATC and Pilot?
Amidst the recent Japan Airlines 516 crash at Haneda and multiple others in history like the famous KLM/PanAm at Tenerife, it has always bothered me that these accidents were so easily preventable since they were caused by a simple misunderstanding/communication.
As we know, most of aircraft accidents occur at the landing/takeoff phase.
Why can't we use digital two way radios with text capability which already exist and are very affordable and have been around for a very long time?
UPDATE: There are many answers that seemed to ignore all the notes below because the notes were added later as a clarification to those answers. The most important thing to clarify(where most of the confusion happened) is that this is NOT a replacement, is only an additional communication tool and that this would only be used for specific common instructions and ONLY ATC would do the initial communication(TextToSpeech), pilot would only push a button to ACKnowledge message .
The radio would have a Text to Speech function that would convert the message to audio so that the pilot doesn't have to look at a screen(this also means cristal clear audio, this will also help on the English language mispronunciations/barriers in many countries). The pilot then would push a button on the yoke/stick which will "readback" the message by sending a MSG ID(XXX) ACKNOWLEDGED (each message sent by ATC can be assigned an ID) .
ATC would have a computer with predefined messages(i.e. TAXI VIA A5, HOLD SHORT, CLEARED FOR TAKEOFF, ETC) which would be transmitted to the radio. This will also make the work of the controller faster, more efficient and clear. This will also prevent assigning two aircrafts conflicting instructions since all instructions would be handled trough the computer which will detect if there is a conflict so that the message is not sent.
Besides helping avoid misunderstandings or not understandings, this will also have extra benefits:

Prevent from talking over.
Allows to re-read the last instruction (i.e: in case you are talking with your co-pilot) without using the voice frequency to clarify wasting precious talk space specially on congested areas.
It can also prevent from guessing (i.e. if your talking with your copilot) and ATC sends a message where you think you heard your call sign but it was just another aircraft with a similar call sign)

NOTE: The pilot/ATC would still be able to communicate with voice if they choose to(ie to discuss a special situation) with the same radio they already have.
NOTE2: This is only for communication at the Landing/Takeoff phase which would be within the distance/limit of digital radio transmission.
NOTE3:  The pilot doesn't have to look at a screen at all since it will always have TextToSpeech, he/she only has to push a button on the yoke to ACKnowledge the message. If the pilot is unable or wants something else he/she can use the radio with voice normally(no SpeechToText).
NOTE4: For situational awareness purposes, the radio could be configured to TextToSpeech other aircraft instructions that can be cause of a collision . (i.e.: Instructions like Clear for Take Off/land, Cross Runway, Line up and Wait, etc)  or it can be configured to Speech all aircrafts instructions if the pilot wishes. (In addition it is worth noting that in theory a conflicting instruction shouldn't happen because the controller will input each instruction into the pc and then converted to audio)

For some context, in the United States in the past ten years there have been a total of three people killed in airline accidents out of over 80 million commercial airline flights. And the last U.S. fatal accident caused by runway incursion was at LAX in 1991, there have been over 200 million flights since then. I'm not saying that ongoing vigilance isn't needed, but you would need to prove that what you are proposing will be more safe than the current system. People are not infallible, but neither is technology.

Digit text-messaging communication for ATC is becoming more common: CPDLC. It's used for enroure communication only, however. The text format is far better than voice for a lengthy reroute, but for real-time direction, it's not going to work. Having one or both pilots looking down at the screen when they should be eyes-out taxiing the aircraft... not a good scenario. And the current state of the part for speech to hex isn't good enough to reply along, even without issues caused by accents.

I will push back strongly against your assertion that text is faster or easier than voice from an ATC perspective. A radio transmission is one physical action (keying the mic) and then just speaking! Doing that as text would require selecting the recipient, selecting the message, and pressing Send—and that's if absolutely nothing needed to be modified/customized.

You are not thinking this through. ATC will have a button that says "TURN X"? No, they will have TURN 10, TURN 20, TURN 30, etc, all the way through TURN 360. Then ASCEND 3000, ASCEND 4000, ASCEND 5000. Then DESCEND 3000, DESCEND 4000, DESCEND 5000. That is a long list to scroll through. Or else they will have to type in a custom value each time. This will not be faster than simply keying the microphone and saying the message.

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

We are not - not exclusively, anyway.
ACARS has been operational for over 40 years by now. No reason to use text-to-speech and reverse, though - if you go text, go text all the way. We have a question on this SE explaining ACARS and related concepts. This article goes into technical details of the protocol and its deficiencies.

Other than converting text to speech, ACARS does exactly what you propose. It communicates in predefined, fixed-format text messages. Every modern airliner has an ACARS terminal, usually located as shown above (A320). Text can be printed out as well, to keep a record.
Voice communication with ATC is a faster, real-time channel* of communication. Two different channels provide dissimilar redundancy - the crew can hear from ATC, as well as read their text messages. Text is better than speech for some things, such as being able to check it again, rather than having to remember. Radio readback is an additional check, where the controller verifies if they've been understood correctly.
Even all two or three safeties fail sometimes. But if there was only one channel of communication, we'd probably have more incidents. This does double the workload, but this is intentional.
ACARS has evolved somewhat over the years, from a simple text screen to a large color display with a QWERTY keyboard:

Using text-to-speech would reduce the overall system's reliability. Humans can only process one speech channel at a time, but we can read and listen simultaneously (to some extent). It's less likely that the same error will occur separately through two distinct channels than through the same one.
Single-button ACK is not practical. Under high workload, simpler tasks get offloaded to Cognitive System 1. System 1 (common to humans and animals) is needed for fast snap reactions, picking the most obvious decision. If you normally press the ACK button 99% of the time, it becomes the automatic reaction in case of cognitive overload. Such reactions have caused multiple crashes.
A verbal readback engages Cognitive System 2, which is verbal thinking, unique to humans, slow and deep. This ensures at least some measure of understanding. To match it with text, one would need to use typeback, but typing incurs a higher cognitive load than speech.
Ultimately, it's a balancing act between cognitive load, speed, and reliability. TTS voice + ACK button could save time and workload, but be very prone to false positives. Text+voice+readback+typeback, separately by PF and PM, would offer maximum reliability, but take the most time (as well as blind-typing literacy and a sidestick). The current system of readbacks plus text information is a compromise that mostly works.

Therac, thanks for bringing ACARS point. However I'm not proposing to get away from audio radio communication I'm only proposing an addition to the radio. Neither I'm proposing STT(only TTS). And I'm not suggesting a ALL text or ALL audio  approach as you yourself brought up on your post for obvious redundancy reasons. Please see NOTE3 on my post.

@GabrielRodriguez I see your edit. I'll address the reasons why TTS is not used in the answer.

To match it with text, one would need to use typeback And where voice requires pressing one button, typeback requires using at least one hand, commonly two - you can type with one finger but it takes a long time - and you don't have that time in an emergency situation.

Lets take a few of your points in order

As we know, most of aircraft accidents occur at the landing/takeoff phase and runway incursion is a very common issue.

The first part is correct, the second part is citation needed. Compared to the number of flights per year, I'd argue that the number is tiny and most of them are minor issues without substantial safety implications. I'm not going to do a full analysis but wiki has a bunch of info on runway incursions. A small excerpt:

Between October 2016 and September 2017, 1,341 [runway incursions] were reported [to the FAA]. Of these, six were placed in the most serious categories A and B. Four of these were considered ATC incidents, and two were "pilot deviations".

The FAA website reports 10,000,000+ movements per year. So 6 serious incidents for over 10 million movements.

Why can't we use digital two way radios

We could, but unfortunately the cost of refitting every radio is prohibitive, and although no doubt beneficial, like everything in business its a cost:benefit analysis.

The radio would have a Text to Speech function ... help on the English language mispronunciations/barriers in many countries

While text to speech has come on in leaps and bounds over the last decade or two. It still struggled with strong accents (obligatory "eleven" video link). So now, instead of misunderstood audio where a controller or pilot can ask for clarification, we have misunderstood text were it could be completely unclear if itts a misunderstanding or a mis-read of Text-to-speech. Add to this the fact that every pilot in the world must be able to use passable English, and they still would in a text-to-speech regime, where is the benefit?

The pilot then would push a button on the yoke/stick which will "readback" the message

This is the part of your question that makes least sense. The point of a readback is not just to acknowledge receipt, it is specifically to read back the receipt so the controller confirms that the pilot has correctly received the message and no miscommunication has occurred. If this becomes automated, there's no way of knowing if the message was sent AND received correctly. I appreciate that with digital communication there are less avenues for misunderstanding, but it still makes no sense to automatically acknowledge a message.

This will also prevent assigning two aircrafts conflicting instructions since all instructions would be handled trough the computer which will detect if there is a conflict so that the message is not sent.

This part makes a lot of sense, but probably aside from all your other suggestions. Albeit the uptake of the aviation industry to new technology is slow due to regulatory demands, I think in the next decade or so we will see AI being introduced to ATC functions, and probably for the purpose of error detection.

I apologize, I didn't meant that runway incursion was a cause of most accidents, I meant that there are a lot of "incidents" of runway incursion. I disagree with your point about misunderstanding text, I think is way more easy to misunderstand or not understand at all someone by voice than by Text(ToSpeech), especially a foreign person. Also knowing that most of these instructions will be already written, the ATC operator would just have to click a button, for other instructions/situations it  would use normal audio.

@GabrielRodriguez But there arn't "a lot of incidents". In the example I gave it was 0.013% of movements overall, and 0.00006% serious incidents. And exactly 0% injuries for a number of years.

@Jaimec, please ignore the "runway incursion". I have removed it from the original post.

Oh please let's not let AI into ATC functions! That's far too scary!! (Coming from a guy who has spent 30 years in IT.)

@FreeMan I think it's sort of inevitable, not to perform critical functions, but to try to detect anomoly - which AI is notoriously good at.

In Australia we have the ability to communicate in text form with pilots using CPDLC (Controller Pilot Data Link Communications) and send Pre-Departure Clearances (PDC) via text. I know this is available around the world and not what you have stated but this will reenforce what @randomhead has been saying about the issues with text communications over verbal communications.
So, I am going to answer this in parts based on your statements in the questions.
The Benefits

Besides helping avoid misunderstandings or not understandings, this will also have extra benefits:

Prevent from talking over.
Allows to re-read the last instruction (i.e: in case you are talking with your co-pilot) without using the voice frequency to clarify wasting precious talk space specially on congested areas.
It can also prevent from guessing (i.e. if your talking with your copilot) and ATC sends a message where you think you heard your call sign but it was just another aircraft with a similar call sign.

Yes, this can reduce the risks of helping avoid misunderstandings.
But the talking over each other is replaced with overload on the ATC with the ability for all aircraft to make requests at the same time. I have experienced this with CPDLC, CPDLC sends messages by satellite between the aircraft and the ATC. With each request coming in there is more workload on the controller. You might say the workload always existed but that is not the case.
When you can't hear the other pilots you, pilots can make requests that they may not have made on a busy frequency. Now the ATC has to process that request individually.
Additionally, when you are in congested airspace, I expect you to be paying attention as the instructions that are given are important to be enacted promptly. Most pilots I know have a higher attention in those times to get their instructions. Additionally, I don't mind the occasional Say Again.
The final benefit can be all about callsign confusion. This is can also happen on the ATC side with instructions being sent to the wrong aircraft. So, your replacing one issue which is occasionally a problem which we have procedures to reduce the risk. And replace with a new risk that can happen at any time even when there is no callsign confusion.
The Notes
Note

NOTE: The pilot/ATC would still be able to communicate with voice if they choose to(ie to discuss a special situation) with the same radio they already have.

This is still the case where there is ability to do text back and forth right now and is chosen by both ATC and pilots' majority of the time to be the primary form of communication. The only time I have ever chosen to do this via text-based systems is when there is a complex information to issue (e.g. amended route, pre-departure clearance, weather information)
Note 2

NOTE2: This is only for communication at the Landing/Takeoff phase which would be within the distance/limit of digital radio transmission.

That is the time when lots of things are happening and most text communication systems like CPDLC are not used. The ability for quick information exchange is essential. It is more useful during cruise and currently used over when HF communication is needed in the form of CPDLC.

Note 3

NOTE3: The pilot doesn't have to look at a screen at all since it will always have TextToSpeech, he/she only has to push a button on the yoke to ACKnowledge the message. If the pilot is unable or wants something else he/she can use the radio with voice normally(no SpeechToText).

This would only be safe if implemented identically to CPDLC, this is that by pushing that ACK button the message is translated by the flight computer and the computer flies the new clearance. If a human needs to interact with the aircraft, then there has to be a readback.
Correcting readbacks is the most common thing I have done in the form of fixing errors in my time as an ATC. Internally of the ATC centre we even readback between the controllers that are sitting right next to me on a recorded line so that everyone is sure that both parties have the same information. Humans make mistakes and the readback is critical in catching the first layer of that mistake.
Note 4

NOTE4: For situational awareness purposes, the radio could be configured to TextToSpeech other aircraft instructions that can be cause of a collision . (i.e.: Instructions like Clear for Take Off/land, Cross Runway, Line up and Wait, etc) or it can be configured to Speech all aircrafts instructions if the pilot wishes. (In addition it is worth noting that in theory a conflicting instruction shouldn't happen because the controller will input each instruction into the pc and then converted to audio)

The situational awareness lost if aircraft are not able to hear what each has been a part of the reason for incidents in the past. One of those is the crash between PSA182 and a Cessna both were on approach but on different frequencies though not the main reason for the crash it was a contributing factor. A simple google search can find more.
Comments
These are some of the comments I want to address directly:
From Gabe:

@randomhead I strongly disagree with you. Remember ATC will have a predefined list of common instructions(TURN X, ASCEND X, DESCEND X) they will not write anything they will just click a button on the computer. The pilot won't write or talk anything he/she would just ACK with the push of a button. They will talk (with voice) only if unable/something else. See NOTE 3

The number of different messages for a standard interface exists for CPDLC in the Australian ATC system, this system was populated with all the normal needed messages and came out with over 300 different messages. This was without the different levels built in. This required a multiple stage menu just to find the correct message. To give simple climb instruction in enroute is a minimum of 6 clicks.

Select the aircraft
Select the communication capability for that aircraft
Select the level change section
Select the level message to send
Select the level to send with the message (or type it in which is a minimum of 4 keystrokes)
Click send

This is slow and clunky.
From Gabe:

@randomhead I disagree with you again. As ATC, remember you have to take into consideration that the pilot also needs to read back the message you just transmitted, and also remember that on each message you and the pilot have to add the call sign, which makes the transmission even longer. If you add all that up, I guarantee you text is going to be much faster. Regarding the point example ASCEND 3000, ASCEND 4000, etc, messages like these are going to be more like a template: ASCEND ___ (the controller would just have to make two clicks, 1st Template, 2nd Custom value)

The adding of the callsign verbally is replaced by me having to read your callsign I know this can be attached to your exact aircraft on my screen but I have since the time of your last message been interrupted by four other aircraft with requests. That means I need to get my mental picture updated with that request is for you and not from someone else.
Conculsion
Air Traffic Controller use the radio to control the speed of information they get from pilots to reduce the risk of overload. But they also use it to get quick information out to aircraft, I can give 5 instructions verbally for each data instruction I give.
It will be a long time until verbal instructions are replaced by text systems as the primary form of communication in approach cells or towers. They will expand first in upper airspace in enroute first but away from major terminal areas that have sequencing. CPDLC started as communication for in the ocean as a replacement for HF which is really, really slow at communication. But it is rarely used where there is VHF coverage because it is easier to give a simple instruction via voice.

@randomhead and others thanks for your comments above that have helped in providing this answer

When I was a student I asked this exact same question. Likely because I was struggling with controlling the aircraft, navigating, and speaking with ATC all at the same time (English is not my first language).
Now that I am used to it, I'd much rather just talk to them than to fumble around with a keyboard to try to send a message. Also, it helps with situational awareness for everyone to hear each other. Now, if we had neuralink or similar technologies, then I'd be interest ;)

as I explained multiple times, as a pilot you don't have to fumble around anything or type any or click anything nor do you need to look at any screen. You only need to REJECT or ACKNOWLEGE the instruction. with the push of a button in the yoke. To reject you would talk by voice normally as you are doing currently, And you will always be able to talk by voice if you want. Regarding situational awareness, these feature could be added so that the TextToSpeech also reads the messages sent to to other pilots(but that would be an option if you want it on or not).

@GabrielRodriguez But you can't just remove the readback as a concept. It's there for a reason. That would add so much risk.

@60levelchange you use readbacks currently because the information is transmitted via audio in analog way, which can be misunderstood. You wouldn't need a "readback" if the information is transmitted digitally in text format(the controller wouldn't make a "typo" either in the computer because these will be predefined instructions).

No, readbacks are not just used to correct for bad audio reception, but also to ensure that the receiving pilot understood the instructions. The controllers will make mistakes, if messages are predefined they will select the wrong message.
If the controller gives a wrong instruction today, everyone on the channel can hear it and correct or make the controller and recipient aware of it. Using digital messages sent to one airplane they wont, and the chance of something going wrong will be higher than it currently is, especially if the receiver doesn't understand the message.

Controllers sometimes catch their own mistakes as well, when hearing a pilot do a (correct) readback of their instruction, and realising it's not actually the instruction they intended to give.


text:
Which engine is more financially economical to operate, a turboprop or gasoline engine?
I've seen a discussion about this somewhere before. I'm aware that there's no real way to calculate this. So rather then comparing weight and volume for efficiency, let's compare prices. Which engine is more economical to operate? What price will get you further along?
(sorry in advance if the answers depend on weight or volume)

Gasoline piston engines are more economical than turbines. Duplicate question: https://aviation.stackexchange.com/questions/29588/are-turboprops-more-efficient-than-piston-engines-thrust-per-fuel-consumption

I saw that question a while back, but I couldn't find anything on the economics of it

One-for-one is a rather poor comparison here, the whole package must be considered for efficacy.
It would be more appropriate to compare the economics of operating two 2000shaft hp turbines to four 1000hp piston engines.
This is because the turbines are far more than twice as reliable. So any flights over substantial amounts of water or other areas without suitable runways will require a plane with 4 piston engines but only 2 turbine engines for similar risk mitigation.
The reliability aspect is different from durability or TBO, reliability is chance of an engine failure in the middle of any given flight.
Especially when you start to factor in reserve power, because the non-failed engine[s] will need to increase output after one fails.
This reliability is also why modern designs tend to progress one piston, two piston/oneturbine, two turbine; and you haven't seen 4 piston-engine planes since turbines moved out of the experimental stage. Even 2 piston-engine designs are slowly losing favor to single turbines.
Add to this the improved economics of high altitude flight. Especially regarding direct thrust turbines, which have even better high altitude performance, reliability, and simpler maintainence than turbo props.

Piston engines are significantly less expensive to operate, as long as the aircraft's performance and reliability requirements can be satisfied by a piston engine.
It's not just the fuel, it's the maintenance. Turbines run longer between overhauls and rarely fail, but the overhaul costs tens of thousands, and more if repairs are required. There just aren't any cheap parts or cheap repairs to make.
Even though older carbureted designs have poor fuel economy, small turbines (under 1000 bhp) are even worse in that regard. As turbines get larger, they get more efficient, but also more expensive, even per kW, both to buy and to maintain. Modern fuel-injected piston engines are the cheapest to own, with tolerable fuel consumption (helped by supporting lead-free car gasoline) and tolerable maintenance cost. Not considering electric aircraft, which promise the lowest costs ever, but are too new to judge decisively.
The five cheapest turboprops will all cost over $100k/year just for owning them, no flying hours included.
Comparing apples to apples, this site cites USD 1100/hour for the Cessna 208, one of the cheapest turboprops to fly. Let's look at some comparable pistons and turboprops:

The piston-powered Cessna 421C, which is actually faster than the 208, is USD 800/hour. The Cessna 425 - a turboprop version of the 421 - costs USD 1700/hour to fly.
The piston-powered Beechcraft Queen Air 80 is USD 900/hour, while the King Air 90 turboprop (slightly faster, but the same range and capacity) is USD 1,500/hour.
The Piston Beaver is USD 700/hour and the otherwise identical Turbine Beaver is USD 1100/hour.
That's a 50%+ premium for a small turboprop over a comparable piston engine equipped aircraft.

Turboprops simply cost more: to buy, to own, to fly. They offer better performance, reliability, and lighter engine weight. They also scale up well. After the initial premium for switching to turbines, cost per seat starts to go down.
There is a limit beyond which piston engines don't work well: they can't fly very high, they don't scale up well, and they aren't reliable enough for a common carrier airline. So there's no modern piston equivalent to commercial turboprops. But turboprops don't scale down well, since a smaller one still needs lots of expensive precision engineering.
As a result, within the size and performance envelope where piston-powered airplanes are still a thing, they are more economical. This is despite the higher cost of less energy-dense avgas. Diesels would've been the ultimate fuel-sippers, but they remain rare for now, with electrics taking up the efficiency niche.
NB: This answer applies to the kinds of aircraft that commonly fly today and where a choice between piston and turboprop engines is possible. Large aviation piston engines now survive only as historical items, like in warbirds.

From a fuel burn perspective, these ballpark numbers can be useful to remember:
Specific Fuel Consumption, Expressed as LBS/HP/HR:

Turboprops - .5 to.7 lbs traditionally, mid .4s to .5 for newer designs with electronically trimmed, and later FADEC, fuel control.
like the P&W PW100 series used in Dash 8s and ATRs.
Two Stroke Gas - Similar to older TPs, About .6
Four Stroke Air Cooled Carbureted - About .45
Four Stroke Air Cooled Fuel Injected - Low .4s
Four Stroke Fuel Injected Liquid Cooled - High .3s
Diesels - Low to Mid .3s

Using .45 for a typical Lycoming carbureted engine, you can work out quickly that a 150 hp engine, cruising at 75% power or 112 hp, times .45, is burning 50.6 lb/hr, or just over 8 US Gal/hr, which is about what you normally see on those engines in the real world.
A particular class of aircraft gas engines, the turbocompound radials like the Wright R 3350, used supercharging plus Power Recovery Turbines to recover about 300 HP from the exhaust flow. These engines had SFC's in the high .3s, approaching diesels. The Canadair Argus used 3350s because of the incredible endurance they could provide, although unfortunately the choice became a problem later in life because of the engines' reliability issues.
Piston engines are much cheaper to run on direct operating costs, but when you factor in other variables, like reliability, weight, smoothness, TBO, obsolescence issues, availability of kerosene compared to gasoline, turboprops usually win out from a business case perspective once you exceed, say, 500 hp, because you can just add a little more kerosene to get the same range and avoid the large piston engine grief.

That, and JET-A smells better than AvGas does!

Turboprops become about as economical as pistons once you move up to thousands of horsepowers. Only the small ones (like the venerable PT-6) consume as much as 6 lbs per hp and hour.

@PeterKämpf  I don't think so Peter.  No turboprop could ever come close to the SFC of a R3350 or even an R2800. When Buffalo Airways, star of that TV show Ice Pilots, started using Electras to replace their DC4s and C46s, the much higher fuel use of the Electra was something they had to accommodate in their business case. The scarcity of gasoline in the arctic tipped the scales.  PT-6s do rather well. IIRC their SFC is in the mid/high 5s.

Both engines consume about 230 g/kWh when run at their optimum. Now compare that to the Progress D-27 or the Europrop TP400: Their 240 g/kWh aren't so different. Of course, when compared with turboprops of the same period, the pistons win hands down. But technology has progressed since then.

So then why are diesel engines in planes so rare?

@ROIMaison Diesels share the main issues of gasoline engines, such as vibration and limited reliability, and they are even heavier. Perhaps if there was no large existing installed base of avgas engines...

@PeterKämpf I stand corrected. I checked P&W PW 100 series data, being one of the most widely used high power TPs and they started at about .5 lb/hp/hr, and the newest version, the PW 150 is .43, about level with carbureted gas 4 strokes. I modified my answer.

@ROIMaison Mostly it's been weight and structural limits. To get the power to weight ratio of a gasoline aircraft engine, diesels have to boost the intake charge pressure to high levels, with supercharging.  This has posed structural problems for years, to make cylinders light enough, and only recently had the metallurgy progressed to where you could build a diesel as light as a gas engine for the same power and certify it. They are making inroads, but slowly, because they are expensive and the entrenchment of Lyc/Cont.

@RalphJ I had some finger trouble with decimal places. All of the values are fractions of a pound. Point 6, point 45, point 4 etc.  As an overall range, one third to two thirds of a pound/hp/hr/.

All clear now, thanks!

Up to about 450 horsepower, piston engines are more economical for the typical use model (short hops, slow speeds, unpressurized aircraft, low gross weights).
Above 450 horsepower, turbines are more effective (better power/weight ratio, longer TBO times, ready availability of spares) for the use model (longer stage lengths, higher speeds, pressurized planes, higher gross weights).
"Little" turbines are really uneconomical because turbine engines do not scale down as well as they scale up, so pistons still rule for Cessna 150s and 172s. "Big" piston engines are uneconomical because there's so much that can go wrong with the machinery. For example, the biggest commonly-used piston engine had 28 cylinders, 4360 cubic inch displacement, and 56 spark plugs. Fouling the plugs by running rich on the ground  or an improper start sequence required a 6-hour maintenance gig to pull out and clean them all per engine.  Those engines went away quickly once turboprops  came on line.

I am curious where your 450hp figure came from. The 1200hp P&W 1830 is a very reliable, economical engine to operate. I would say that even with larger horsepower engines, piston engines are still more economical. In addition to being cheaper to build, they burn less fuel, which translates to more payload, and more profit. There are lots of reasons why high horsepower piston engines fell out of favour, but I don’t consider operational economy to be a reason.

Ugh, again downvoting without the slightest hint of an explanation. People, please, it's rude. If you're casting the first downvote, leave a word, a hint, something that people can use to understand your displeasure. It does no good to the rest of us to guess at your motivations.

The only answer with the crucial information about scale (even though the 450 hp is too low, specific fuel consumption only matches for turbopros of 10 times the power). +1! What is still missing is that turboprops become more economical if run day in, day out. If the plane sits mostly in a hangar, pistons are cheapest. But once you need to pay for the more expensive fuel and run them for >1000 hours per year, the turboprop is impossible to beat.